TCAM (ternary content addressable memory) is an important class of memory device widely used for internet networks. These elements work on matching input data with stored address data. One feature of such elements is that they require storing 3 distinct states 1, 0 and “don't care”. Normal implementations of such a device require a very large number of transistors to enable such functions and this leads to extremely large die sizes.
A typical implementation of a static random access memory (SRAM) TCAM cell consists of a ternary storage containing two SRAM cells which combines ten to twelve transistors. It also has comparison logic, which is basically a XNOR gate using four additional pass transistors. Hence very large cells size of fourteen to sixteen transistors, hence a costly device. Conventional TCAM cells are often provided as a two standard SRAM cells with four or more transistors designed to implement the exclusive-OR (EOR) function.
Unlike a RAM chip, which has simple storage cells, each individual memory bit in a fully parallel TCAM has its own associated comparison circuit to detect a match between the stored data bit and the input data bit. TCAM chips are thus considerably smaller in storage capacity than regular memory chips. Additionally, match outputs from each cell in the data word can be combined to yield a complete data word match signal. The associated additional circuitry further increases the physical size of the TCAM chip. Furthermore, CAM and TCAM as it is done today (using SRAM elements) is intrinsically volatile, meaning that the data are lost when the power is turned off. As a result, every comparison circuit needs being active on every clock cycle, resulting in large power dissipation. With a large price tag, high power and intrinsic volatility, TCAM is only used in specialized applications where searching speed cannot be accomplished using a less costly method.
Emerging memory technology and high-speed lookup-intensive applications are demanding ternary content addressable memories with large word sizes, which suffer from lower search speeds due to large cell capacitance.